Process for manufacturing a vibrating type transducer

ABSTRACT

A vibrating type transducer wherein an H-shaped vibrator, formed integrally with a silicon substrate and having a hollow chamber provided therearound, is kept self oscillating at its natural resonance frequency together with an amplifier. A physical quantity, such as force, pressure, differential pressure, or the like, which is applied to the silicon substrate is detected by a change in the natural frequency arising at the vibrator corresponding to the physical quantity. The invention also includes a method for manufacturing such transducer using a semiconductor technique, including steps for keeping a vacuum internally in the hollow chamber, imparting an initial tension to the vibrator, and then operating the amplifier to have stable self oscillation.

This is a division of application Ser. No. 245,681 filed 9/16/88, U.S.Pat. No. 4,926,143.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to a vibrating type transducer and manufacturingprocess thereof; and more particularly, to a vibrating type transducerwhich is capable of generating stable self-oscillation and has high S/Nratio, and to a process for manufacturing such transducer.

2. Description of Prior Art

FIGS. 1, 2, 3 and 4 are block diagrams depicting one example of theprior art vibrating type transducer, wherein FIG. 1 is a perspectiveview of the transducer which is used as a pressure sensor, FIG. 2 is ablock diagram wherein section A of FIG. 1 is enlarged and a vibrationdetection circuit is connected thereto, FIG. 3 is a sectional view takenalong line A--A of FIG. 2, and FIG. 4 is an explanatory drawing showingan electrical equivalent circuit of the device of FIG. 2.

FIG. 1 depicts a silicon single crystal substrate 10 having a (100)plane on the top thereof, which is 10¹⁵ atoms/cm³ or below, for example,in impurity concentration and of p-type conduction. A diaphragm 11 isformed from the back, through etching, as a thin layer on one side ofsubstrate 10, as depicted.

A peripheral thick wall part 10 of diaphragm 11 is joined to a pedestal14 having a pressure hole 13 at the center. Pedestal 14 has a pressurepipe 15 joined thereto so as to communicate with pressure hole 13. Apressure P (see arrow) to be measured, is introduced to pressure pipe15.

An n⁺ diffusion layer (not indicated) having 10¹⁷ impurity concentrationis formed partly on a surface of the side of diaphragm 11 indicated byreference letter A which is not etched. As shown in FIG. 2, a vibrator16 is formed on a part of the n⁺ diffusion layer in the direction of<001>. Vibrator 16 is obtained, for example, by processing the n⁺ layerand the p-layer formed on diaphragm 11 using photolithography andunderetching.

A magnet 17 is provided over vibrator 16 almost at the center thereoforthogonally to vibrator 16 and also positioned to not be in contacttherewith. As shown in FIG. 3 an SiO₂ film 18, used as an insulatingfilm, is provided on layer 11, as depicted.

Metallic electrodes 19a, 19b (see FIG. 2), such as , for example, A1 andthe like, are depicted with one end of electrode 19a being connected tothe n⁺ layer extending from vibrator 16, through a contact hole 20aprovided by way of SiO₂ layer, and with the other end of electrode 19abeing connected through a lead wire (unnumbered) to a comparisonresistance R_(o), almost equal to the resistance value of vibrator 16,and also to an input end of amplifier 21. An output signal is generatedfrom an output end of amplifier 21, which is connected to one end ofprimary coil L₁ of transformer 22. Another end of coil L₁ is connectedto common.

The other end of comparison resistance R_(o) is connected to one end ofa secondary coil L₂ of transformer 22 with the midpoint thereof beingconnected to common, and the other end of secondary coil L₂ beingconnected to the n⁺ layer through metallic electrode 19b and a contacthole 20b formed likewise on another end of vibrator 16.

In the above device, when a reverse bias voltage is applied to theinsulation between the p-type layer (i.e substrate 10) and the n⁺ layer(i.e vibrator 16), and an alternating current is carried to vibrator 16,an impedance of vibrator 16 rises in a resonance state of vibrator 16.If the impedance is R, the equivalent circuit of FIG. 4, is obtained.

Secondary coil L₂ having a center point C_(o) connected to common,comparison resistance R_(o), and impedance R_(o) together constitute abridge. Thus, if an unbalanced signal, due to the bridge, is detected onamplifier 21 and the output is fed back positively to primary coil L₁through feedback line 23, the system will generate a self-oscillation ata natural vibration frequency of vibrator 16.

The impedance R of vibrator 16 rises at the natural vibration frequencyand may be expressed by the following:

    R≈(1/222)·(1/(Egγ).sup.1/2)·(AB.sup.2 l.sup.2 /bh.sup.2)·Q+Rd (                        1)

wherein E is the modulus of elasticity, g is gravity acceleration, γ isthe density of the vibrator material, A is the constant determined byvibration mode, B is the magnetic flux density, l is the length ofvibration beam, b is the width of vibration beam, h is the thickness ofthe vibrator beam, Q is the quality factor, and Rd is the DC resistancevalue.

According to equation (1), since Q of vibrator 16 takes values ofseveral hundreds to several tens of thousands, a large amplitude signalis obtainable as an output of amplifier 21 in the resonance state. Thus,by making the gain of amplifier 21 sufficiently large and by providingpositive feedback, the system of the vibrating type transducer is selfexcited to vibration at the natural vibration frequency.

A p-type device obtained from diffusing, for example B (boron), on ann-type silicon substrate at 4×10¹⁹ atoms/cm³ or more, through selectiveetching may be used as a vibrator.

However, in such vibrating type transducer, a counter electromotiveforce generated on vibrator 16 is detected from an unbalanced voltage ofthe AC bridge. Since the component of an excited current cannotthoroughly be suppressed by the DC bridge, a voltage according to theexcited current component is multiplied by the bridge output. Thus, theS/N ratio of the output is deteriorated by change in impedance of thevibrator being superposed on voltage of the excited component. Hence,stable output signal is not obtainable.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to overcome theaforementioned and other deficiencies and disadvantages of the priorart.

Another object is to provide a vibrating type transducer having avibrator which has a satisfactory S/N (signal to noise) ratio, has astable output, and has high sensitivity, and to provide a process formanufacturing such transducer.

The foregoing and other objects and features are attained by theinvention which encompasses a vibrating type transducer comprising avibrator body comprising a silicon single crystal material which isprovided on a silicon single crystal substrate; an excitation means forexciting the vibratory body; a vibration detection means for detectingvibration of the excited vibrator body; an H-shaped vibrator body havingtwo first vibrators with both ends fixed on a substrate, which aredisposed in parallel with each other and second vibrator for couplingmechanically center portions of the first vibrators; a magnetic fieldimpression means for impressing a DC magnetic field orthogonally to thevibrator body; an excitation means for vibrating the vibrators by mutualaction with the DC magnetic field by flowing an alternating current toopposite ends of the one first vibrator or to the one same end of thetwo first vibrators; a vibration detection means for detecting anelectromotive force generated on opposite ends of the other firstvibrator or on the other same end of the two first vibrators; and anamplification means connected between the excitation means and vibrationdetection means.

The vibrator is provided with a predetermined initial tension byimplanting another atom having a coupling radius which is smaller thanthe coupling radius of an atom constituting the vibrator.

The process for manufacturing the vibrating type transducer comprisesforming a beam like vibrator integrally on a thin diaphragm formed on asilicon single crystal substrate through a predetermined gap with thediaphragm except the end portion; covering the top thereof with a shellthrough a predetermined gap with the vibrator; forming a partcorresponding to the gap and a vibrator consisting of silicon or silicondioxide integrally with a substrate; covering an upper portion of thegap corresponding part with a shell equivalent part integrally with thesubstrate; forming an etching reagent injection port reaching the gapcorresponding part on the shell equivalent part to remove the gapcorresponding part through etching; and then closing the injection portto maintain air tightness.

In the invention, if a physical quantity, such as an external force, isapplied to the diaphragm on the substrate, a natural vibration frequencyof the vibrator body changes according to then applied external force. Avibration of the vibrator body is detected by the vibration detectionmeans and a change in the natural frequency is extracted as an outputsignal Then, the physical quantity applied to the diaphragm is detectedfrom the change in natural frequency.

Furthermore, in the manufacturing process, the thin diaphragm is formedon the silicon substrate through an etching process, and the H-shapedvibrator body can be formed on this portion integrally with thediaphragm by use of an etching process and use of semiconductortechniques according to the characteristics of the single crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view depicting a prior art vibrating typetransducer used as a pressure sensor.

FIG. 2 is a block diagram depicting an enlargement of section A of FIG.1 and a vibration detection circuit connected thereto.

FIG. 3 is a sectional view taken along line A--A in FIG. 2.

FIG. 4 is a circuit diagram depicting an electrical equivalent circuitof the FIG. 2 device.

FIG. 5 is a general block diagram depicting an illustrative embodimentof the invention.

FIGS. 6(a) and 6(b) main parts of the vibrator body of FIG. 5, whereinFIG. 6(a) is a top view with the shell removed, and FIG. 6(b) is asectional view taken along B--B of FIG. 6(a).

FIG. 7 is a graph depicting S/N ratio characteristics of the transducerof FIG. 5.

FIG. 8 is a block diagram depicting another illustrative embodiment ofthe invention.

FIGS. 9(a)-9(f) are drawings depicting the process for manufacturing theembodiment of FIG. 5.

FIGS. 10(a) and 10(b) are drawings depicting a part of the process forforming the H-shaped vibrator body of FIG. 5.

FIGS. 11(a)-11(f) are drawings depicting a process for enhancing andstabilizing the yield of the vibrator in the process shown in FIGS.9(a)-9(f).

FIG. 12 is a drawing depicting an improvement of the process of FIGS.9(a)-9(f).

FIG. 13 is a drawing for illustrating the effect of the auxiliaryepitaxial layer obtained in FIGS. 11(a)-11(f).

FIGS. 14(a)-14(c) are drawings depicting the main part of themanufacturing process for obtaining a vibratory body which keeps theshell vacuum interiorly.

FIG. 15 is a characteristic graph depicting the effect of temperature ondissociation pressure in extracting gas to keep the shell vacuuminteriorly in the process of FIGS. 14(a)-14(c).

FIGS. 16(a) and 16(b) are drawings depicting a modification of theprocess shown in FIGS. 14(a)-14(c).

FIG. 17 is a sectional view depicting the main part of a vibrating typetransducer wherein an initial tension is applied to the vibrator.

FIG. 18 is a table depicting the relations between a covalent bondradius R_(i) of each impurity with the ratio of covalent bond radiusR_(i) of various impurities and covalent bond radius R_(si) of silicon.

FIG. 19 is a graph depicting relation between change of lattice constantand impurity density.

FIGS. 20(a)-20(g) are views depicting a main part of the manufacturingprocess for the vibrator body which is a main part of the vibrating typestrain sensor of FIG. 17.

FIG. 21 is a circuit diagram depicting details of the amplifier shown inFIG. 5.

FIG. 22 is a characteristic graph depicting the effects of the amplifier9 of FIG. 21.

FIG. 23 is a characteristic graph depicting the effect of a circuit whenthe field effect transistor of FIG. 21 is removed and short-circuited tomake the driving force constant.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 5, 6(a) and 6(b) depict a vibrator body 24 comprising an H-shapedvibrator comprising first vibrators 26A, 26B and second vibrator 27 ofp-type material, such as silicon, which are formed integrally on adiaphragm 25 made of a silicon single crystal of n-type. As in the caseof the diaphragm 11 shown in FIG. 3, diaphragm 25 is formed by etchingand thinning the central portion of a lower surface of the n-typesilicon substrate having a thick wall part (not indicated) therearound,and is displaced as a whole from having a measuring pressure appliedthereon. An H-shaped recess 28, in which first and second vibrators arecontained, is formed through an etching process on a part of crystalface (100) on an upper surface of diaphragm 25.

Beamlike first vibrators 26A, 26B are formed of p-type materialintegrally with diaphragm 25 in parallel with a crystal axis <001> eachdisposed over recess 28 (see FIG. 6(b). These central portions arecoupled by p-type beamlike second vibrator 27 rectangularly to thesevibrators, thereby forming an H shaped vibrator, as depicted.

Electrodes 29 and 30 are formed on opposite ends of first vibrator 26A,and electrodes 31 and 32 are formed on opposite ends of first vibrator26B.

A magnet 17 is disposed on an upper portion of second vibrator 27 inparallel therewith, thus generating a magnetic field rectangularly onfirst vibrators 26A, 26B.

An output terminal of an input transformer 33, functioning as anexcitation means, is connected to electrodes 29, 30. One end of inputterminal 34 is connected to an output terminal 35 and the other end isconnected to common.

An input terminal of an output transformer 36, functioning as avibration detection means, is connected to electrodes 31, 32 and outputterminals 37, 38 are connected to input ends of amplifier 39. The outputof amplifier 39 is connected to terminal 35 and fed back to terminal 34.

The shell covering an upper part of the diaphragm 25 is excluded forsake of convenience of description. However, as will be describedhereinafter, first vibrators 26A, 26B and second vibrator 27 arepractically covered therearound integrally with the diaphragm 25 througha predetermined gap using a semiconductor technique such as epitaxialgrowth or the like. Moreover, the gap is kept in a vacuum internally tomaintain a high Q factor to vibration of the vibrators.

In the above embodiment, first vibrator 26A is excited to vibrate by avoltage inputted to input transformer 33 from amplifier 39 according tomutual action with the magnetic field of magnet 17. The vibration thenvibrates first vibrator 26B through second vibrator 27, and thevibration drives the output transformer 36 to generate an electromotiveforce e on the input end through mutual action with magnet 17.Electromotive force e is inputted to amplifier 39 through outputtransformer 36, then amplified, and then extracted at output terminal35. The amplified voltage is fed back positively to the inputtransformer 33, which is repeated then to self oscillation of thesystem.

As described above, vibrator body 24 is divided into first vibrator 26Afor excitation and first vibrator 26B for electromotive force, and firstvibrators 26A and 26B are coupled together mechanically at loops ofvibration by second vibrator 27. Thus, the excited current component isnot superposed on the electromotive force e, and a high excitedcomponent removing ratio (i.e. the S/N ratio) is obtained.

In FIG. 7, the abscissa indicates a frequency of 1 KHz per graduation,and the ordinate indicates an attenuation of 5 dB per graduation. Theresonance frequency, when no pressure is applied to diaphragm 25, is71.551.1 Hz. The point indicated by a mark x is -13.3 dBm with areference level as -7.0 dBm, coming gradually near to the lineindicating a -52 dBm noise line accordingly as it comes off theresonance point. The S/N ratio is represented as a difference of thesefigures. Thus, the S/N ratio coming in at 30 to 40 dB is far better thanever previously obtained.

In FIG. 8, a secondary side of input transformer 33 is connected to thesame ends of first vibrators 26A, 26B and the primary side of outputtransformer 36 is connected to the opposite ends of first vibrators 26A,26B.

In the above embodiment, second vibrator 27 is of a p-type silicon.However, it is not necessarily limited thereto, and may be, for example,a conductor, such as aluminum or the like evaporated on silicon oxide(SiO₂) or silicon nitride (Si₃ N₄).

Furthermore, such vibrating type transducers change their vibratingfrequency according to the temperature coefficient of the elasticmodulus of silicon Thus, these can be utilized as a thermometer, whencontained in a vacuum vessel, and also as a densimeter other thanpressure gauge.

As described, vibrator body 24 is divided into first vibrator 26A forexcitation and first vibrator 26B for detecting the electromotive force,and furthermore, the first vibrators 26A,26B are coupled togethermechanically by second vibrator 27 at the loops of vibrations. Thus, anexcited current component is not included, and a high excited componentremoving ratio (i.e. S/N ratio) is obtained.

Consequently, according to the embodiment of FIG. 8, a vibrating typetransducer satisfactory in S/N ratio and stable in output signalfrequency is obtained by the invention.

FIGS. 9(a)-9(f), for simplicity of description, refer to manufacturingbeam like first vibrator 26A without vibrators 26B and 27.

FIG. 9(a) shows a process for forming a protective coat and opening onepart thereof, wherein a protective coat 41, such as silicon oxide,silicon nitride or the like, is formed on a crystal face (100) of ann-type silicon single crystal substrate 40. Then, an opening 42 isformed on a part of protective coat 41 by means of a mask with a patternin the shape of the first vibrator 26A formed thereon.

The process then proceeds to FIG. 9(b) wherein a recess is formed in thesubstrate Recess 43 is formed in substrate 40 corresponding to opening42 by etching in hydrogen chloride in an atmosphere of 1,050° C.hydrogen (H₂). In this case, an anisotropic etching may be used by meansof an alkali solution at 40° C. to 130° C. , for example, instead ofhydrogen chloride.

FIG. 9(c) shows an epitaxial growth process wherein hydrogen chloride ismixed in a source gas in an atmosphere of 1,050° C. hydrogen to form amultilayer selective epitaxial growth. In this respect:

1. For the first step, a first epitaxial layer 44, functioning as alower half of the gap corresponding part, is subjected to selectiveepitaxial growth on recess 43 by means of p-type silicon having animpurity concentration of 10¹⁸ cm⁻³ boron.

2. For the second step, a second epitaxial layer 45 corresponding to thefirst vibrator 26A is subjected to selective epitaxial growth on asurface of the first epitaxial layer 44 so as to close the opening 42 bymeans of p-type silicon having impurity concentration of 10²⁰ cm⁻³boron.

3. For the third step, a third epitaxial layer 46, functioning as anupper half of the gap corresponding part, is subjected to selectiveepitaxial growth on a surface of second epitaxial layer 45 by means ofp-type silicon having an impurity concentration of 10¹⁸ cm⁻³ boron.

4. For the fourth step, a fourth epitaxial layer 47, corresponding tothe shell, which will be described hereinafter, is subjected toselective epitaxial growth on a surface of third epitaxial layer 46, bymeans of p-type silicon having an impurity concentration of 10²⁰ cm⁻³boron. In this case, n-type silicon having an impurity concentration of10¹⁷ cm⁻³ phosphorus may also be used for the third epitaxial layer 46.

FIG. 9(d) shows a process for the forming of an injection port throughwhich an etching reagent is injected, wherein protective coat 41 isetched and removed by means of hydrofluoric acid (HF), and an injectionport 48, through which an etching reagent is injected, is provided on aside of the fourth epitaxial layer 47.

FIG. 9(e) shows a selective etching process for forming a clearancebetween the vibrator and the substrate and other elements, wherein apositive pulse voltage is applied, from a pulse supply E_(p), so thatn-type substrate 40 will be reverse biased to p-type fourth epitaxiallayer 47. An alkali solution is injected through injection port 48 toprotect substrate 40. Thus, first epitaxial layer 44 and third epitaxiallayer 46 are removed through selective etching. In this case, n-typesilicon having an impurity concentration of 10¹⁷ cm⁻³ phosphorus may beused for the third epitaxial layer 46, and p-type silicon having animpurity concentration of 10²⁰ cm⁻³ boron may be used for the fourthepitaxial layer 47. The phenomenon, that etching action is suppressedwhen boron concentration exceeds 4×10¹⁹ cm⁻³, is utilized therefor.

FIG. 9(f) shows the process for sealing, wherein n-type silicon issubjected to an epitaxial growth in an atmosphere of hydrogen at 1,050°C., and an epitaxial layer 50 is formed on the outer surfaces ofsubstrate 40 and fourth epitaxial layer 47, to construct a shell 51partly, and to close injection port 48 thereby to effect sealing. Thesealing process may also comprise (1) closing the injection port 48 byheat oxidation, (2) closing the injection port 48 by filming theinjection port 48 with polysilicon according to the CVD process or bysputtering, (3) filling up silicon in the injection port 48 according tovacuum evaporation of the epitaxial process, or (4) filling up aninsulating material, such as, for example, glass (SiO₂), siliconnitride, alumina, or the like, in the injection port 48 according thethe CVD process, by sputtering , or by evaporation.

While not so indicated, diaphragm 25 may be formed thereafter byreducing the thickness of the substrate through etching from a bottomside of substrate 40.

Advantageously, the foregoing process realizes the following effects.Since substrate 40, second epitaxial layer 45, functioning as firstvibrator 26A and shell 51 are formed integrally, it is not necessary tobond substrate 40 and shell 51 together, thus avoiding instability whichresults from bonding. Moreover, the air and the vibrators can beisolated by a simple structure, miniaturization can be easily carriedout. Furthermore, since semiconductor techniques are used, accuratepositioning, thicknesses and shapes of the vibrators and shell areobtained.

FIGS. 10(a) and 10(b) show a part of the process for forming theH-shaped vibrator body, wherein the process shown in FIGS. 10(a) and10(b) may be substituted for those steps shown in FIGS. 9(a) and 9(b) torealize H-shaped vibrator body 24. First, as shown in FIG. 10(a), aprotective coat 52, such as silicon oxide, silicon nitride, or the like,is formed on an upper surface of crystal plane (100) of siliconsubstrate 40. Then, protective coat 52, formed on a surface of substrate40, is removed to form an H-shape through use of photolithography with amask having an H-shaped opening, thus forming an H-shaped opening 53 onprotective coating 52. The H-shaped opening 53 is disposed so thatH-shaped beams formed by each first vibrators 26A,26B and secondvibrator 27 face in the direction <001> of substrate 40 and also in thedirection orthogonal thereto.

Next, as shown in FIG. 10(b), an H-shaped recess 54, corresponding toopening 53, is formed on substrate 40 by etching protective coat 52having opening 53. Then, H-shaped vibrator body 24 shown in FIG. 5 isformed according to the process of FIGS. 9(c) 9(f).

FIGS. 11(a)-11(f) show a process for enhancing and stabilizing the yieldof vibrators in the manufacturing process shown in FIGS. 9(a) -9(f),wherein the process is almost the same as the process shown in FIGS.9(a)-9(f) excepting for the step of FIG. 11(c). The process of FIG.11(c) comprises forming an epitaxial layer of p⁺⁺ p-type material havinga boron impurity of high concentration, which layer material 71 has athin thickness of 1 μm or below, on a surface of recess 43 formed asshown in FIG. 11(b). In this case, the impurity concentration is setpreferably to the limit of etching the p-type epitaxial layer 71 withetching reagent, or, for example, at 3×10¹⁹ cm⁻³ or so.

The process then shifts to that for etching of FIG. 11(b) throughepitaxial process of FIG. 11(d) and that for forming an etching reagentinjection port of FIG. 11(e), wherein an etching reagent is injectedfrom injection port 48 to etch and remove first epitaxial layer 44equivalent to the gap corresponding part and the third epitaxial layer46. In this case, auxiliary epitaxial layer 71 is of p-type and is highin impurity concentration inherently. Thus, it is not etched. However,since it is very thin, the boron impurity concentration deteriorates andis ready for etching by an alkali solution according to autodoping usinga selective epitaxial process and diffusion at a heating process. Thus,an n-type face of substrate 40 comes out on the surface.

The foregoing process will now be further described with reference toFIG. 12 and FIG. 13. In the process of FIG. 11(c), where there is noauxiliary epitaxial layer 71 present, there remains an island like partof p-type silicon on a pn junction between n-type substrate 40 andp-type first epitaxial layer 44 using the etching process of FIG. 11(f)

A p-type residue 72 (see FIG. 12) which remains as an island like part,forms an n-type inversion layer 73 inverted to n-type at a boundary withthe alkali solution which is an etching reagent during etching. Thus, apath, through which a current flows from pulse supply E_(p) (see FIG.11(f), as indicated by an arrow, is formed to protect a surface ofresidue 72 from etching (which may cause the lower portion of thevibrator to be partly not etched. Thus, auxiliary epitaxial layer 71,which is a p-type material having high p⁺⁺ concentration of impurities(3×10¹⁹ cm⁻³ or so) of boron dope layer having a thin thickness of 1 μmor less, is formed on top of substrate 40. The leakage current il isinterrupted to keep residue 72 from being formed and stable etching isattained, thus enhancing productivity.

The following process is for forming the shell, as in the case of FIG.9(f), as described in FIGS. 14(a)-14(c) wherein the process produces avibrator body which keeps vacuum internally within the shell. Fordetecting pressure with high sensitivity and with high Q factor, thevibrator should be in a vacuum. In this case, the process forms the beamlike vibrators 26A, 26B, 27 integrally with diaphragm 25. FIG.14(a)-14(c) first vibrators of FIG. 5 as being in vacuum. The process ofFIG. 9(a)-9(e) remains the same and the etching result of FIG. 10(a)which is equivalent to FIG. 9(f) is obtained. In the process of FIG.14(b), the outer surfaces of substrate 40 and fourth epitaxial layer 47,are subjected to an n-type epitaxial growth at a temperature of 1,050°C. generally in an atmosphere of hydrogen or in vacuum. Injection port48, formed between substrate 40 and fourth epitaxial layer 47, is filledby epitaxial growth, and shell 51 is thus formed. The vibrator body fora vibrating type transducer having, for example, first vibrator 26A, isformed of the second epitaxial layer internally. In this case, an n-typelayer equivalent in thickness to a clearance (t) of injection port 48 isformed around first vibrator 26A and also on the inside of a hollowchamber 74.

In the process of FIG. 14(b), since epitaxial growth is effected in anatmosphere of hydrogen, hollow chamber 74, formed between substrate 40of a silicon single crystal and shell 51, is charged with hydrogen.

As shown in FIG. 14(c), a vibrating type transducer, having the vibratorbody, is put into an atmosphere kept at vacuum at a temperature of 900°C. and hydrogen is extracted to vacuum through a crystal lattice ofsilicon. The degree of vacuum thus obtained is 1×10⁻³ Torr or less.

Then, a similar result was obtained in inert gas and nitrogen gas withless hydrogen partial pressure.

Next, the hydrogen extraction will be described with reference to FIG.15, wherein the abscissa indicates temperature, and the ordinateindicates dissociation pressure. The straight line drawn obliquely fromthe point of origin indicates a boundary separating a domain whereinhydrogen is absorbed in silicon of the substrate 40 and a domain whereinit is extracted externally from silicon. According to the graph, whenleft as it stands in vacuum/at a temperature of T1 or, for example,1,200° K. for a long time, hydrogen within shell 51 is absorbed into thesilicon of shell 51 and substrate 40 and diffused thereinto, andhydrogen having reached the surface is dissociated and discharged if thepressure is P₁, or for example, 10⁻³ Torr or less. Thus, hollow chamber74 may be retained at the degree of vacuum of, for example, 10⁻³ Torrinternally. The above can be understood from the results obtained fromcarrying out a test according to the above process wherein a Q factor of3×10⁴ or more of the first vibrator 26A, which corresponds to about 10⁻³Torr, was obtained using hollow chamber 74 within shell 51.

FIGS. 16(a) and 16(b) depict a process which is a modification of theprocess shown in FIG. 14(a)-14(c), wherein the process up to FIG. 14(a)remains the same, and the process then shifts to that of FIG. 16(a).While injection port 48 is formed through etching in the process of FIG.14(a),the process 16(a) is used for sealing the injection port 48.

In the process, oxygen is substituted in a gap formed by fourthepitaxial layer 47, working as first vibrator 26A to second epitaxiallayer 45, and silicon substrate 40. Then, injection port 48 is sealedthrough sputtering amorphous silicon, to thereby form shell 75.

Then, the process shifts to FIG. 16(b), wherein the vibrating typetransducer, including the vibrating body, is placed in a vacuum at atemperature of 900° C. or more, and an inside wall of hollow chamber 74is oxidized by oxygen filled in hollow chamber 74 using the process ofFIG. 16(a), or oxygen in the silicon is diffused to come out of thesilicon surface partly, thereby stepping up the degree of vacuum.

According to the above process, vibrators are formed integrally with thesilicon substrate with a predetermined gap left therein, and then, avacuum is realized through a predetermined process. Thus, a vibratingtype transducer which is superior in both pressure and temperaturecharacteristics is realized.

FIG. 17 depicts a vibrating type transducer wherein an initial tensionis applied to the vibrators. The vibrator body is constructed such thatthe opposite ends are fixed, for example, on n-type silicon substrate40. The p-type vibrator 13 is fixed with a predetermined gap retained tothe substrate 40 and barring the opposite ends, which are covered bysilicon shell 51 integrally with substrate 40, and hollow chamber 74 isformed surrounded thereby Hollow chamber 74 retains a vacuum internally.

Then, a measuring pressure P_(m), for example, is applied to diaphragm25, and a resonance frequency of vibrator 76, with the opposite endsfixed on diaphragm 25 which corresponds to a strain arising on vibrator76, is measured, to thereby obtain measuring pressure P_(m).

Meanwhile, unless an initial tension is given even at the time whenmeasuring pressure P_(m) is zero, buckling will be caused on vibrator 76by the pressure P_(m), which is not ready for measurement. Unlessdispersion of the initial tension is controlled, dispersion ofsensitivity may also result therefrom. FIGS. 18 and 19 will be used todescribe this effect.

As will be understood from FIG. 18, while the covalent bond radiusR_(si) of silicon is 1.17 Å, that of phosphorus is 1.10 Å and that ofboron is 0.88 Å, which are rather small Accordingly, when boron orphosphorus is injected into silicon, the resulting part is subjected toa tensile strain. From FIG. 19, therefore, where impurity concentrationof boron is 10²⁰ cm⁻³, for example, change of the lattice constant is2×10⁻³⁰ Å. Since the lattice constant of silicon is 5.431 Å, the strainis about 4×10⁻⁴ (equal to 2×10⁻³ /5. 431). For a strain at 4×10⁻⁴ ormore, boron is injected at a double rate or at 2×10²⁰ cm⁻³. Then, aninitial tension at 8×10⁻⁴ is generated in proportion to the injectionrate. Accordingly, an arbitrary initial tension may be given frominjecting an arbitrary concentration of boron An initial tension is thusgiven to vibrator 76 shown in FIG. 17.

For a strain of less than 4×10⁻⁴, a phosphorus concentration of n-typesilicon substrate 40 is increased, or the vibrator 76 is oxidized tosegregate boron on the surface of the vibrator into the oxide film, andfrom removing the oxide film by use of HF, the boron concentration inthe vibrator 76 is decreased to adjust the strain at 4×10⁻⁴ or lessThen, as will be apparent from FIG. 21, it is presumed that the strainwill almost not rise at the boron concentration of 10¹⁷ cm⁻³ or so.

FIG. 20(a) depicts formation of recess 43 through an HC1 etching step inthe process of FIG. 9(a) and FIG. 9(b). Next, as shown in FIG. 20(b), a10¹⁸ cm⁻³ concentration of boron (of p-type) is subjected to selectiveepitaxial growth into recess 43, in an atmosphere of hydrogen, at atemperature of 1,050° C., thereby to form first epitaxial layer 44.Then, as shown in FIG. 20(c), boron (of p-type), adjusted to aconcentration of 10²⁰ cm⁻³ in an atmosphere of hydrogen, at atemperature of 1,050° C., is subjected to selective epitaxial growth onfirst epitaxial layer 44, to thereby form a second epitaxial layer 77working as vibrator 76.

The covalent bond radius of silicon is 1.17 Å, and that of boron is 0.88Å. Thus, when boron is partly injected into silicon, the resulting partis subjected to a tensile strength, which is utilized for imparting thenecessary initial tension thereto through adjusting the boron density ofthe second epitaxial layer 77 working as vibrator 76.

Next, as shown in FIG. 20(d), a 10¹⁸ cm⁻³ impurity concentration ofboron of p-type is subjected to selective epitaxial growth on secondepitaxial layer 77 in an atmosphere of hydrogen and at 1050° C., tothereby form third epitaxial layer 46. Furthermore, as shown in FIG.20(e), a 10²⁰ cm⁻³ concentration of boron of p-type is subjected toselective epitaxial growth on third epitaxial layer 46 in an atmosphereof hydrogen and at a temperature of 1,050° C., to thereby form fourthepitaxial layer 47.

FIG. 20(f) shows an etching process for removing first epitaxial layer44 and third epitaxial layer 46 in the state wherein Si0₂ protectivecoat 41 has been removed (this step is not shown) through etching by useof hydrogen fluoride (HF) after the process for selective epitaxialgrowth shown in FIG. 20(e).

While not illustrated, the whole arrangement may be soaked in an alkalisolution in this etching process, and a positive pulse voltage of 5V inpeak value and 0.04 Hz, or so, in repetition frequency may be appliedfrom DC pulse supply E_(p) so that n-type silicon substrate 40 will be aplus potential to p-type second epitaxial layer 77. Since n-type siliconsubstrate 40 and fourth epitaxial layer 47 have an insoluble film formedon the surface, each to a passive state according to the voltageapplication, the etching rate becomes considerably low to firstepitaxial layer 44 and third epitaxial layer 46, which is utilized forremoving layer 44 and layer 46. Furthermore, when the concentration ofdoped boron is greater than 4×10¹⁹ cm⁻³, the etching rate isconsiderably reduced from that of the normal case where silicon is notdoped. Such a phenomenon is utilized to produce an arrangement whereininjection port 48 is provided partly, and furthermore a gap is securedbetween substrate 40 and layer 77 as a whole, leaving the second layer77 as shown in FIG. 20(g).

The following process is the same as that of FIG. 9(g) and FIGS.14(b)-14(e). The main part of the vibratory body shown in FIG. 17 isformed through such a process. For further adjustment of an initialtension of vibrator 76, a phosphorus impurity concentration in n-typesilicon substrate 40, for example, will be adjusted, thereby to adjustthe initial tension on the relative strain of substrate 40 and secondlayer 77.

Also, the apparent initial tension may be reduced by subjecting a lowconcentration n-type silicon to epitaxial growth on vibrator 76 to asuitable thickness Furthermore, heat oxidation may be used to generate acompression strain in a hot oxide film, to thereby adjust the apparentinitial tension Moreover, the initial tension can be adjusted likewisethrough CVD, sputtering , evaporation or other like processes.

The impurity atom which is injected has been specified as boron orphosphorus However, the invention is not necessarily limited thereto,and the vibrator beam is also not limited to silicon only. The impurityconcentration unit is atoms/cm³, although, in the art the word atoms isusually omitted and such omitted designation is well known.

The invention can be used to measure pressure, such as applied byacceleration and hence may serve as an acceleration meter, also, tomeasure pressure differential, etc.

As described, an initial tension is provided to the vibrator beam in asimple manner and is easily adjusted, as contrasted with the prior art.

Next the amplifier of FIG. 5 will be described with reference to FIG. 21The problem with the prior art device shown in FIG. 4 is that since thevibrator is constructed in such a manner as to oscillate in a nonlineardomain, the oscillation frequency changes from, limiting an amplitudeon, for example, a Zener diode. Also, arrangement used to control thedriving voltage is capable of changing the amplitude of the violatoraccording to boundary conditions of a junction with another resonancesystem or with measuring fluid, and prevents generation of accurateresonance frequency. Such a problem is solved by using the amplifiershown in FIG. 21.

In FIG. 21, there is depicted an amplifier circuit AMC1 with its inputends (+), (-) connected to output ends 37,38 of the vibrator body 24(see FIG. 5). The output end is inputted further to an amplifier circuitAMC2 through a coupling capacitor C₅ and its output voltage is outputtedto a junction J for applying to phase adjusting circuit PHC. The outputof PHC is then applied to a gain adjusting circuit GAC The amplificationoutput of circuit GAC, after amplification at its first stage ,isapplied to resistance R₁₀, field effect transistor Q₁, a series circuitcomprising a transformer T, and an output voltage, controlled formagnitude, is generated at output terminal 35 from the secondary sidewinding of transformer T.

On the other hand, a voltage V_(j) of junction J is inputted to a halfwave rectifier circuit HWR, converted into a DC voltage E_(j),corresponding to the magnitude of voltage V_(j), and then inputted to aninversion input end (-) of a comparator CMP. A reference voltage V_(R)is applied to a non-inversion input end (+) of comparator CMP from anamplitude setting circuit ASC. Comparator CMP amplifies the deviationbetween DC voltage E_(j) and reference voltage V_(R) and applies fromthe output end thereof the differential voltage to the gate of fieldeffect transistor Q₁ to control the resistance between drain and gate,thus controlling the current flowing to the transformer T.

In these circuits, the phase is adjusted by capacitor C₆ and aresistance R₁₇. The amplitude of voltage generated at the output side 35is set by resistance R₂₆.

In the above embodiment, when a voltage is applied to input transformer29 from amplifier 39, a current i flows to first vibrator 26A from theoutput. Thus, first vibrator 26A vibrates on an electromotive forceoperating with a magnetic field of magnet 17. The vibration operates onfirst vibrator 26B through second vibrator 27. However, since a magneticfield is impressed on first vibrator 26B from magnet 17, a voltage e isgenerated on first vibrator 26B and, inputted to amplifier 39 throughoutput transformer 36. Amplifier 39 amplifies the voltage and generatesan amplified voltage at output terminal 35. The amplified voltage isapplied to input transformer 33 and to first vibrator 26A as a greatervoltage.

By repeating the above loop coupling, amplifier 39 and vibrator body 24make a self-oscillation arrangement. Then, by setting the gain of theloop at 1 or more, the self oscillation becomes lasting.

In this case, the voltage amplitude of the self oscillation iscontrolled so as to come within a range of constant error to referencevoltage V_(R). That is, when DC voltage E_(j) corresponding to junctionvoltage V_(j) is great compared to reference voltage V_(R), an internalresistance of field effect transistor Q₁ is increased on the output ofthe comparator CMP according to these deviations, a current flowing totransformer T is minimized, and the voltage generated at the outputterminal 35 is minimized. As a result, the voltage applied to vibratorbody 24 is minimized, and the voltage inputted to amplifier 39 is alsominimized.

On the other hand, when DC voltage E_(j) corresponding to junctionvoltage V_(j) is small compared to the reference voltage V_(R), theoperation is reversed.

Thus, the oscillation amplitude operates to coincide with the referencevoltage V_(R) within the range of constant error. The error isdetermined by the output voltage/gain of comparator CMP. Accordingly,where the gain of the comparator CMP is large, the error may bedisregarded in value, and the amplitude of the vibrator operates to beequal to the reference voltage V_(R) at all times.

Next, the effect when the circuit configuration of FIG. 21 is employedwill be discussed with reference to FIGS. 22 and 23, wherein, FIG. 22indicates the effect when the circuit of FIG. 21 is used and FIG. 23indicates the effect when the prior art circuit is used wherein thefield effect transistor Q₁ of FIG. 21 is removed due to short circuitingand the driving force is kept constant, i.e. the drive from the constantsupply voltage. The span is 1 kg/cm² in either case, and the abscissaindicates pressure, and the ordinate indicates indexed value.

As will be understood from the results, while the fluctuation is ±0.005%or so in the case of FIG. 22, the fluctuation at ±0.025 maximum or so isindicated in the case of FIG. 23. This indicates an improvement of aboutfive times or so.

As described the invention comprises detecting an amplitude of selfoscillation at a point halfway through the amplifier, comparing thedetected amplitude with a preset reference voltage, adjusting a gaincontrolling means provided at the rear stage for the amplitude tocoincide with the reference voltage, thereby retaining the amplitudeconstant, so that the oscillation amplitude is retained constant at alltimes without being influenced by external conditions and fluctuationwill not be brought on the self oscillation frequency. Thus, a highprecision vibrating type transducer is obtained by the invention.

The foregoing description is illustrative of the principles of theinvention. Numerous extensions and modifications thereof would beapparent to the person skilled in the art. All such extensions andmodifications are to be considered to be within the spirit and scope ofthe invention.

What is claimed is:
 1. In a manufacturing process for vibrating typetransducer, wherein a beam which functions as a vibrator is formedintegrally across a recess formed on a thin diaphragm formed on asilicon substrate through a predetermined first gap with said diaphragm,a top thereof being covered with a shell through a predetermined secondgap with said vibrator, the improvement comprising the stepsofintegrally forming a gap corresponding part consisting of silicon orsilicon oxide, which is formed finally into said first gap and secondgap, and said beam forming said vibrator with said substrate, coveringan upper portion of said gap corresponding part with a part equivalentto said shell integrally with said substrate, forming an injection portfor etching reagent which reaches said gap corresponding part on saidshell equivalent part, removing said gap corresponding part throughetching, and closing said injection port for air tightness.
 2. Theprocess of claim 1, wherein said substrate is made of n-type electricconduction type silicon, said gap corresponding part is made of p-typeelectric conduction type silicon, and said vibrator and said shell arerespectively made of p-type electric conduction type silicon having highimpurity concentration.
 3. The process of claim 1, wherein said recessis formed into an H-shaped recess wherein an H-shaped vibrator body isenclosed through etching said substrate by means of an H-shaped etchingmask in the direction of <001> to a crystal plane of said siliconsubstrate being (100) and also orthogonal thereto.
 4. The process ofclaim 1, wherein forming a protective coat of silicon oxide or nitrideon said substrate, partly removing said protective coat through etchingto leave a recess, forming said vibrator and said gap corresponding partand shell equivalent part in said recess by epitaxial growth, andremoving the remaining portion of said protective coat through etchingto form said injection port.
 5. The process of claim 2, wherein formingan auxiliary epitaxial layer of 1 μm or less in thickness, and beinghigh in impurity concentration, and of p-type, on said substrate.
 6. Theprocess of claim 1, wherein removing said gap corresponding part throughetching to form a hollow chamber, sealing said injection port in a gasatmosphere and retained at high temperature, so as to keep said hollowchamber as a high vacuum.
 7. The process of claim 6, wherein hydrogen isused for said gas atmosphere.
 8. The process of claim 6, wherein oxygenis used for said gas atmosphere.
 9. In a manufacturing process for avibrating type transducer, wherein beam shaped vibrator is formedintegrally on a thin diaphragm formed on a silicon single crystalsubstrate, a top thereof being covered with a shell, the improvementcomprising the steps ofintegrally forming a first part, consisting ofsilicon or silicon oxide, and said beam shaped vibrator with saidsubstrate; covering an upper portion of said first part with a secondpart integrally with said substrate; forming an injection port foretching reagent which reaches said first part covering said second part;removing said first part by etching; and closing said injection port forair tightness.
 10. The process of claim 9, wherein said substrate is ofn-type silicon, said first part is of p-type silicon, and said beamshaped vibrator and said shell are both of p-type silicon and high inimpurity concentration.
 11. The process of claim 9, wherein an H-shapedvibrator body is formed by etching said substrate by using an H-shapedetching mask in the direction of crystal axis 001 to a crystal face(100) of the crystal silicon substrate and also orthogonal thereto. 12.The process of claim 9, wherein a protective coat is formed, of siliconoxide or nitride, on said substrate; then said protective coat is partlyremoved by etching, to leave a recess; then said beam shaped vibrator,said first part, and said second part are formed in said recess byepitaxial growth, and then the remaining portion of said protectivecoating is removed by etching to form said injection port.
 13. Theprocess of claim 10, wherein an auxiliary epitaxial layer is formed onsaid substrate, said layer being of 1 μm or less in thickness, and beinghigh in impurity concentration, and being of p-type semiconductormaterial.
 14. The process of claim 9, wherein said first part is removedby etching to form a hollow chamber, then said injection port is sealedin a gas atmosphere and retained at high temperature, and said hollowchamber is maintained at high vacuum.
 15. The process of claim 14,wherein said gas atmosphere is hydrogen.
 16. The process of claim 14,wherein said gas atmosphere is oxygen.